On April 20th this year NASA launched Gravity Probe B (GP-B) to test Einstein’s general theory of relativity. The probe will use gyroscopes to attempt to measure the distortion of space caused by the mass of the earth.
The June issue of Sky and Telescope has some further detail on the construction of the gyroscopes:
[...]These are the most perfect spheres within lightyears, deviating from perfection by no more than the thickness of 40 atoms; only neutron stars rival them [...] The [gyroscope] chamber is fused to a 5.6 inch telescope trained on a specially selected guide star to provide an orientation standard good to 0.1 milliarcseconds [...]
So, that’s four atom perfect spheres aligned to one tenth of one thousandth of one sixtieth of one sixtieth of one degree.
I am not sure which I am more impressed with; the precision of the engineering or the fact that Einstein got it right in 1916 without the aid of even a pocket calculator.

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